1. Field of the Invention
The present invention relates to a liquid crystal device evaluation method and apparatus and, more particularly, to a method and apparatus for detecting impurities mixed in a liquid crystal device.
2. Description of the Related Art
In a liquid crystal device, if impurities which respond to electric fields (to be referred to as field responsive impurities hereinafter) are mixed in, e.g., a liquid crystal, the device performance such as response speed and contrast deteriorates, and the service life shortens. Field responsive impurities are chemical species which move in a device or have power of transporting electric charges upon application of an electric field. The field responsive impurities include protons, organic ions, inorganic ions, compounds having hydrogen bonding capability, compounds having electron transport ability, compounds having large dipole moments, compounds having large polarizabilities, and the like. Therefore, it is indispensable to detect, identify, and determine field responsive impurities mixed in a device and to improve the manufacturing process so as to prevent them from being mixed.
Conventionally, for this impurity evaluation a method of measuring the voltage retention ratio of a liquid crystal device at high temperatures is used. This method allows evaluation of a device in its final state. The method, however, requires much time and labor. In addition, no impurities are identified. For these reasons, it takes time to specify causative materials for impurities and a process in which impurities are mixed in the liquid crystal device.
Furthermore, field responsive impurities originate from both a liquid crystal alignment film and a liquid crystal material. For this reason, evaluation of impurities has been performed for the respective cases. For example, with respect to a liquid crystal alignment film made of a polyimide which is formed using polyamic acid as an alignment film material, the imidization ratio in a film formation process is determined by infrared absorption measurement, or impurities are detected using a change in the anisotropy of infrared absorption of a film. Since these methods use infrared absorption measurement of a liquid crystal alignment film, it takes time to perform the measurement, and the measurement sensitivity is not high enough.